The name for this situation is filtration lines and is normally caused by dust traveling thoughout your home. When your furnace turns on, air is sucked underneath the walls and under doors. The carpet then acts as a filter leaving dirt particles around the edges causing black lines. We have a special procedure to remove these black lines to reduce this problem. To prevent this problem, you will need to change filters frequently and minimize burning candles which can cause soot particles into your carpet. You can also have your air ducts professionally cleaned.